In U.S. Pat. No. 4,113,360 issued Sep. 12, 1978 to Giienter Bauer et al., titled “Indicating Device For Illustrating Symbols Of All Kinds,” a display panel is disclosed comprising a first plate acting as a light guide or fluorescent material, a second plate positioned some distance apart from the first plate, and a thin movable film situated between the two plates. The movable film is flexible and can be made to locally contact the first plate and allow light to be transmitted from the first plate into the film. If the film is constructed to scatter the light, then the movable film acts as a light modulator, or optical switch, to create bright and dark regions on the plates as the film contacts or separates from the first plate, respectively. Rapid contact and separation of the film from the first plate can be used to create gray regions.
Bauer et al. teach controlling the film's movement by electrical means. For example, the film may contain a very thin layer of indium tin oxide that permits an electrical charge to be applied to the film. Similar conductive layers may be placed on the plates. An electrical bias between the plates and the film produces electrostatic forces that move the film toward or away from the light guide. Alternatively, U.S. Pat. No. 5,771,321 issued Jun. 23, 1998 to Ernest Stern, titled “Micromechanical Optical Switch And Flat Panel Display,” describes an electromechanical means of controlling the film's movement, with an electrical bias providing an attractive electrostatic force and the film deformation providing a mechanical restoring force.
Typically, the plates are rigid with a thickness on the order of millimeters and are comprised of clear materials such as glass or hardened plastic. The film, on the other hand, must be flexible and has thickness on the order of a micron. The film may be comprised of resin material such as polycarbonate or polystyrene as suggested by Stem in U.S. Pat. No. 5,771,321, referenced above.
One drawback to the operation of a display panel described above is that the motion of the movable film may be impeded by an air pressure differential in the spaces existing between the film and the plates. To overcome the air pressure differential, undesirably high voltages are required to move the film. In World Intellectual Property Organization Application Publication No. WO 99/28890, by Gerardus Van Gorkom, published on Jun. 10, 1999, and titled “Display Device Comprising A Light Guide,” a means of minimizing pressure differential is proposed whereby the movable film is situated in an evacuated space. Van Gorkom discloses applying a vacuum of preferably less than 10 Torr (0.013 atm) to the chambers inside the display panel. However, a highly evacuated system is difficult to fabricate and is vulnerable to air leakage during the lifetime of the display panel operating at ambient conditions. Moreover, an evacuated system precludes the use of plastic plates in the display panel since plastic materials are permeable to ambient gases such as nitrogen, oxygen, and water. Because rigid glass plates would be required to maintain a vacuum inside the display panel, a flexible plastic display panel is not possible using Van Gorkom's teachings. Therefore, it remains highly desirable to have a movable film display that does not require an evacuated system.
Unlike the movable film devices described by Van Gorkom, most optical micro-electromechanical systems (MEMS) are packaged at near atmospheric pressure, with gaps at the periphery of movable elements providing a gas exit path. As described by Susanne C. Arney et al. in U.S. Pat. No. 5,751,469, issued May 12, 1998, titled “Method And Apparatus For An Improved Micromechanical Modulator,” and U.S. Pat. No. 5,808,781, issued Sep. 15, 1998 (also to Susanne C. Arney et al.), titled “Method And Apparatus For An Improved Micromechanical Modulator,” additional pores, or holes, can be used to improve etching of a sacrificial material and to fine tune the dynamic response by providing additional gas venting. The devices described in '469 and '781 are substantially smaller in total area than movable film displays and are fabricated with semiconductor-like processes, with the pores patterned using photolithography. During the pore patterning process, the sacrificial material provides rigid support for the movable elements. In '469 and '781, the pores are only placed outside of the active optical area to prevent any deterioration in optical performance. The pore geometry (pore size and placement) is chosen to provide optimal damping of ringing that occurs at the resonant frequency of the movable element.
The optical MEMS approach described in '469 and '781 and related art for providing gas-venting pores is, however, not suitable for large-area movable film displays for several reasons.
1. Fabrication Difficulties: Photolithographic patterning is not well suited for forming pores in the movable film, because the film is suspended over a series of spacers without employing a sacrificial layer at all. The suspended movable film is too fragile to allow photolithographic processes and, prior to assembly, the movable film (and any carrier) is too flexible for precise photolithographic exposures. Furthermore, distortion of the movable film would be likely to occur during wet photolithographic process steps, along with an increase in defects and particle contamination. Etch processes are also not well developed for polymeric films.
2. Pore Placement: The active optical area is preferably a very large portion of the movable film area. In addition, for low-cost assembly, it is preferable not to have a critical alignment between the movable film and the spacers. Both of these factors make it impractical to only place pores outside the active optical area.
3. Performance Criteria: The primary electromechanical performance parameters for large-area movable film displays are switching time and operating voltage, not optimal damping of ringing.
There is a need therefore for a movable film display that does not require an evacuated system, that is manufacturable and that has good optical performance combined with a fast switching time and low operating voltage.